Hybrid electric vehicles (HEV) can save a significant amount fuel, compared with conventional vehicles. A HEV has at least two power sources: one heat engine and at least one electric motor, and can be driven either by electric motor(s) or by the engine together with the electric motor(s).
A widely used hybrid electric drive is called parallel hybrid electric drive train with a single motor. This kind of hybrid electric drive train comprises a heat engine, an electric motor, a power coupling mechanism and a mechanical transmission. Usually, the motor is between the engine and the transmission, and the power coupling mechanism is between the engine and the motor.
The engine is to provide power to the vehicle, and it may be turned off when coming to a stop or during electric driving. The motor has many functions, including driving vehicle by itself, assisting engine to drive, starting the engine, generating electricity, and applying regenerative braking, etc. The power coupling mechanism has functions of engaging/disengaging engine shaft to the transmission. The function of mechanical transmission is to change the speed ratio between the input shaft and the output shaft, so that the engine and motor can work efficiently.
Dual clutch transmissions (DCT) are wildly used in hybrid electric vehicles as the transmission. A DCT transmits power through highly efficient intermeshing gear sets and usually exhibit excellent fuel economy. The synchronizers used in DCT have low spin losses and also contribute to overall operating efficiency. The overall powertrain efficiency also benefits from the flexibility for ratio selection in design.
U.S. Pat. No. 9,308,907 publish a hybrid electric drive system, using DCT as the transmission. The DCT has two clutches: one is connected to the first input shaft and the other is connected to the second input shaft. The electric motor is located between the engine shaft and the two clutches; there is a disconnecting clutch between the motor and the engine shaft. In electric driving mode, the disconnecting clutch is disengaged, and the engine is off and disconnected from the powertrain; the motor works and outputs torque through the gearbox to drive the wheels. When the engine is working, the disconnecting clutch is engaged, connecting the engine to the transmission, and the engine outputs power through the gearbox to drive the wheels. This hybrid transmission is efficient and powerful.
However, DCTs have a few unique design considerations. For example, during a vehicle launches, the dual clutch has to deal with relatively large amount of heat generated by friction. In order to avoid overheated, a dual clutch assembly usually is of large size and takes a large volume of space. Another consideration is how to get more gear ratios for a DCT. Transmissions tend to have more and more speeds or gear ratios if practicable and affordable. Some automatic transmissions with 9 or 10 speeds have come into market. For a DCT, one gear set is usually needed for one more speed, so the number of gear sets has to be increased if more speeds are desired. Having more than 8 speeds will result in a large length of transmissions, and it may cause some issue of packaging the transmission into the engine cabin.